Described herein are ink jet inks that can be cured via at least two different polymerization routes, and methods of forming an image with such inks by advantageously utilizing the different polymerization routes. Also described are ink jet inks that can be cured by a single polymerization route and that contain two photoinitiator systems that absorb radiation at different wavelengths.
The volume of digital color printing is expected to experience significant growth in the coming years. The color images provided by ink jet printing inks are overwhelmingly preferred in panel studies over other digital imaging systems. There is also a strong case to be made that the total cost of ownership of an ink jet printer will ultimately be cheaper than similar volume electrophotography units.
Ink jetting devices are known in the art, and thus extensive description of such devices is not required herein. As described in U.S. Pat. No. 6,547,380, incorporated herein by reference, ink jet printing systems are generally are of two types: continuous stream and drop-on-demand. In continuous stream ink jet systems, ink is emitted in a continuous stream under pressure through at least one orifice or nozzle. The stream is perturbed, causing it to break up into droplets at a fixed distance from the orifice. At the break-up point, the droplets are charged in accordance with digital data signals and passed through an electrostatic field that adjusts the trajectory of each droplet in order to direct it to a gutter for recirculation or a specific location on a recording medium. In drop-on-demand systems, a droplet is expelled from an orifice directly to a position on a recording medium in accordance with digital data signals. A droplet is not formed or expelled unless it is to be placed on the recording medium. There are three types of drop-on-demand ink jet systems. One type of drop-on-demand system is a piezoelectric device that has as its major components an ink filled channel or passageway having a nozzle on one end and a piezoelectric transducer near the other end to produce pressure pulses. Another type of drop-on-demand system is known as acoustic ink printing. As is known, an acoustic beam exerts a radiation pressure against objects upon which it impinges. Thus, when an acoustic beam impinges on a free surface (i.e., liquid/air interface) of a pool of liquid from beneath, the radiation pressure which it exerts against the surface of the pool may reach a sufficiently high level to release individual droplets of liquid from the pool, despite the restraining force of surface tension. Focusing the beam on or near the surface of the pool intensifies the radiation pressure it exerts for a given amount of input power. Still another type of drop-on-demand system is known as thermal ink jet, or bubble jet, and produces high velocity droplets. The major components of this type of drop-on-demand system are an ink filled channel having a nozzle on one end and a heat generating resistor near the nozzle. Printing signals representing digital information originate an electric current pulse in a resistive layer within each ink passageway near the orifice or nozzle, causing the ink vehicle (usually water) in the immediate vicinity to vaporize almost instantaneously and create a bubble. The ink at the orifice is forced out as a propelled droplet as the bubble expands.
In a typical design of a piezoelectric ink jet device, the image is applied by jetting appropriately colored inks during four to six rotations (incremental movements) of a substrate (e.g., an intermediate transfer member) with respect to the ink jetting head, i.e., there is a small translation of the printhead with respect to the substrate in between each rotation. This approach simplifies the printhead design, and the small movements ensure good droplet registration.
In certain ink jet devices, including piezoelectric devices, it is desirable to employ transfuse, i.e., a transfer and fusing step, in forming the image. Transfuse plays an important role in piezoelectric ink jet printers by enabling a high quality image to be built up on a rapidly rotating transfer member. This approach simplifies the print head design, while the small movements of the head ensures good droplet registration. Transfuse typically involves jetting the ink from the ink jet head onto an intermediate transfer member such as a belt or drum, i.e., a transfuse member. This allows the image to be rapidly built onto the transfer member and then subsequently transferred and fused to an image receiving substrate.
Hot melt inks typically used with ink jet printers of the aforementioned type utilize a wax based ink vehicle, e.g., a crystalline wax. Such solid ink jet inks provide vivid color images. These crystalline wax inks partially cool on the intermediate transfer member and are then pressed into the image receiving medium such as paper. Transfuse spreads the image droplet, providing a richer color and lower pile height. The low flow of the solid ink also prevents show through on the paper.
In particular, the crystalline wax inks are jetted onto a transfer member, for example, an aluminum drum, at temperatures of approximately 130-140° C. The wax based inks are heated to such high temperatures to decrease their viscosity for efficient and proper jetting onto the transfer member. The transfer member is heated to approximately 60° C., so that the wax will cool sufficiently to solidify or crystallize. As the transfer member rolls over the recording medium, e.g., paper, the image comprised of wax based ink is pressed into the paper.
However, the use of crystalline waxes also has at least two shortcomings. First, the printhead must be kept at least at 130° C. throughout printing with the device. Moreover, if the printhead is cooled and re-warmed, a lengthy purge cycle that consumes significant amounts of ink must be carried out. Thus, use of crystalline wax inks requires high amounts of energy, an increasingly valuable commodity. Second, the brittle crystalline waxes do not provide robust images and are easily scratched. This is because wax based inks generally crystallize at temperatures greater than room temperature and therefore, the wax based ink that has been transferred to the recording medium is essentially as hard as it will get. The high energy consumption, waste of expensive ink during purging, and fragile images all cause customer dissatisfaction, and in some markets prevents sales penetration.
Xerox Corporation discovered that curing ultraviolet (UV) photosensitive ink jet inks by photoinitiation of the reactive inks can provide tough, permanent images on an image receiving substrate. See, for example, U.S. Pat. Nos. 6,561,640 and 6,536,889, each incorporated herein by reference in its entirety. These patents describe processes of forming ink jetted images using such UV curable inks. Co-pending Application Ser. No. 11/034,850, filed Jan. 14, 2005, now U.S. Pat. No. 7,270,408 entitled “Low Level Cure Transfuse Assist for Printing with Radiation Curable Ink” , incorporated herein by reference in its entirety, describes ink compositions for use in such processes. Co-pending Application Ser. No. 11/034,714, filed Jan. 14, 2005, now U.S. Pat. No. 7,691,920 entitled “Ink Jet Ink of Functionalized Waxes” , incorporated herein by reference in its entirety, describes ink compositions in which the ink vehicle comprises a wax monomer chain functionalized to include at least one reactive group curable upon exposure to radiation of an appropriate wavelength.
UV photocurable inks can be designed to have low viscosity and avoid the need to heat the printhead beyond what may be required for thermal stability. However, such low viscosity inks may be difficult to transfuse, the ink droplets may coalesce during drum rotation, and the low viscosity ink may show through an image receiving substrate, e.g., paper. The aforementioned Xerox Corporation patent properties describe conditions under which such inks may be partially cured on the transfer surface. Such partial cure increases the viscosity of the ink, thus preventing droplet coalescence and image show through. These partially cured, thickened inks have been successfully transferred to image receiving substrates, the viscous ink image possessing sufficient flow to adhere well to paper. Once transferred to the image receiving substrate, the ink image undergoes a final cure to achieve a hard, well adhered final image.
In the aforementioned patent properties, the partial curing is described to be performed via exposure to ultraviolet radiation. For example, the ink is partially cured to a desired rheology via a limited exposure to the UV radiation, with final curing being achieved by longer exposure to the UV radiation, or by utilizing different photoinitiators that have different UV sensitivity ranges. There is a window of optimum rheology to which the ink is desirably partially cured, where the ink drop on the transfer surface is sufficiently viscous to not flow and coalescence with its neighbors, yet can be flattened and spread to achieve the desired image quality and conform to the paper surface so that upon the second and final photocure, a robust, well adhered image is obtained. To achieve this rheology by controlling the length of exposure requires very controlled exposure processes and a very wide rheological window. A challenge is also the potential multiple exposures that an ink droplet may be subjected to during the image build up on the transfer member surface. The member may make 4-6 small incremental rotations as the ink is applied on each pass before transfer to the image receiving substrate occurs. If one envisions a constant illumination of the rotating transfer member, then the first droplets may experience up to six times the exposure of the last set of droplets, and therefore be cured to too great an extent prior to transfer to the image receiving substrate, resulting in reduced image quality and permanence.